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The effects of the reduction of the Ag content and the addition of In on the mechanical properties, soldering characteristics and reaction behavior at solder/package interfaces are investigated in this study. It was found that the addition of In significantly improved the wettability of the solder at reflow temperatures ranging from 230 o C to 240 o C. Moreover, the addition also improves the elongation of the solder alloy, thereby increasing the toughness of the alloy. With the optimization of the In and Ag contents, a Sn-Ag-Cu-In quaternary alloy would be a strong candidate to replace the Sn-3.0Ag-0.5Cu composition.

Through the use of a wetting balance technique, the wetting characteristics of Sn-1.2Ag-0.5Cu-xIn quaternary solder alloys with respect to the In content and soldering temperature were investigated to validate the applicability of compositions with a low Ag content as solder material. It was found that a small addition (0.4-0.6 wt.%) of In significantly improved the wetting properties of the Sn-1.2Ag-0.5Cu-xIn composition at soldering temperatures ranging from 230°C to 240°C due to the excellent wetting property of In. In an observation of the interfacial reaction, it was found that the added In element did not participate in the interfacial reaction with a Cu or Ni pad, unlike in the Sn-Ag-Cu-In case, which has a high In content. The package or boardside IMC layers in Sn-1.2Ag-0.5Cu-0.4In joints were thinner than those of Sn-3.0Ag-0.5Cu, especially after aging.

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